Two large studies in the African- and Hispanic-American communities are providing interim data, and here is what the data say: the prevalence of open-angle glaucoma (OAG) is far higher in these populations than that observed in Caucasian Americans. African-Americans were already considered to have a fourfold higher risk of glaucoma relative to non-Hispanic whites, a finding established years ago in the Baltimore Eye Survey, and subsequently corroborated. 1 The prevalence of OAG among Latinos is now known to be as high, if not higher, than in the black population.

Why? “We do not know,” said Rohit Varma, MD, MPH, principal investigator for the Los Angeles Latino Eye Study (LALES), which was conceived to measure the prevalence of OAG and ocular hypertension (OHT) in a Latino population. “We suppose that either the optic nerves of Latinos are more susceptible to intraocular pressure or that vascular factors contribute to an increased risk compared to persons of European ancestry.”

Similarly, Christopher Girkin, MD, MSPH, can’t explain why African-Americans are at a much higher risk for glaucoma than white Americans. In the black population, primary open-angle glaucoma is more common, more resistant to treatment, more severe and results in a higher prevalence of blindness, he said. “Even controlling for socioeconomic status and access to health care, the discrepancies remain,” said Dr. Girkin, who is an investigator in the African Descent and Glaucoma Evaluation Study Group (ADAGES).

For the past seven or eight years LALES and ADAGES have been tracking their respective populations. The studies will continue for another three or four years.

LALES

Sponsored by the NEI and the National Center on Minority Health and Health Disparities, LALES is a population-based, cross-sectional study involving 6,357 subjects age 40 and older.2

LALES was designed to measure the prevalence of eye disease in Latinos, who are expected to make up 25 percent of the U.S. population by 2050. Most of the LALES participants are of Mexican heritage, so extrapolation of the data to Latinos with other origins is challenging, Dr. Varma said. But the findings have important public health implications, since 60 percent of all Latinos in the United States are Mexican-American, he said. The study thus far has reported:

Latinos have a prevalence of OAG (4.74 percent) comparable to those of non-Hispanic U.S. blacks and significantly higher than non-Hispanic whites. They have a similarly high prevalence of OHT (3.56 percent).

It is estimated that more than 410,000 Latinos may have OAG and over 301,000 may have OHT in one or both eyes.

Seventy-five percent of Latinos with OAG, and 75 percent with OHT, were previously undiagnosed.

Seventeen percent of Latinos in the study had been treated for OAG, and 23 percent for OHT.

Older age was a risk factor, but gender was not.

“I was surprised at the prevalence of glaucoma,” said Dr. Varma, who is professor of ophthalmology and director of the glaucoma service and ocular epidemiology center at the University of Southern California in Los Angeles. The single most important risk factor for undetected glaucoma may be the lack of health insurance, said Dr. Varma, referring to the fact that 30 percent of all participants with eye disease lacked health care coverage.

Get out your sphygmomanometers. “Check the BP” is the clear message of the LALES report on the relationship between blood pressure, perfusion pressure and the prevalence of OAG in the study population.3

LALES found that IOP was higher in persons with higher blood pressure. And it found low perfusion pressure, low diastolic blood pressure, and high systolic and mean arterial blood pressures are associated with a higher prevalence of OAG. “What the LALES paper says is that low blood pressure or very high blood pressure is bad for you,” Dr. Varma said. Also, a higher prevalence of glaucoma was seen with low perfusion pressure, which is blood pressure minus IOP. The vascular theory of the pathogenesis of OAG suggests that low blood pressure, particularly in the face of elevated IOP, can reduce perfusion pressure at the optic nerve head, causing ischemic damage to the retinal ganglion cells.4

Dr. Varma said the vascular hypothesis is compelling because Latinos have a high prevalence of a number of vascular problems, including retinopathy. Dr. Varma advised ophthalmologists to be mindful of the vascular aspects for all patients, not just for Latinos, “because it is the balance of blood pressure and eye pressure, which now multiple, multiple studies, including ours, have shown to be very important.” He added that physicians pay attention to that, particularly in patients with low IOP who continue to get worse. Prostaglandins are a reasonable first-line treatment for those patients, he said, because they don’t lower perfusion pressure significantly.

RACE: An Imprecise Proxy

In 2001, M. Roy Wilson, MD, MS, challenged the medical community to explain why differences in disease burden occur among the races. That challenge, he said, was “to separate the overwhelming influences of underlying socioeconomic differences from the biologic, physiologic and morphologic differences that exist between racial groups and that may contribute to the observed ophthalmologic pathophenotypes.”1

Dr. Wilson, who is chancellor of the University of Colorado in Denver, said it is likely that race, upon which ADAGES is designed, and ethnicity, which LALES is considering, may have less to do with the risk for glaucoma than other factors.

For example, the OHTS found that thinner corneas are a risk factor for glaucoma. And ADAGES found that blacks have thinner corneas. Perhaps, said Dr. Wilson, thin corneas are more important as an isolated factor than race, per se. “That’s simplistic, but it points toward the fact that there may be some other issues, whether biologic, physiologic” that explain the differences.

Dr. Girkin agreed. “The reasons that make African-Americans more vulnerable probably affect people of all races,” he said. “After ADAGES, hopefully we’ll be able to look at what are the characteristics of a nerve in all individuals that increase the propensity for glaucoma to worsen. Instead of relying on broad categories that tend to segregate by race, you can look specifically at the genetic makeup of the person,” he said. “Race is just a proxy. It’s a very imprecise proxy, but for the time being it’s still useful to think in terms of race.”
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1 Wilson, M. R. Ophthalmology 2001;108(10):1719–1720.

ADAGES

This observational cohort is designed to obtain data on visual function and optic nerve structure for the eyes of Americans of African ancestry and compare it with those of European descent. No other study has followed this high-risk population with advanced imaging and functional testing, which optimizes the detection of disease and progressive glaucomatous injury.

“What we’re really looking at in ADAGES is, what are those optic nerve differences that make African-Americans more vulnerable to disease?” said Dr. Girkin, who is professor of ophthalmology at the University of Alabama in Birmingham and director of the glaucoma service there.

In its first report, on healthy eyes:

The African descent (AD) group had greater optic disc area on HRT (2.06 mm²) and OCT (2.47 mm²), and a deeper HRT cup than whites.

Retinal nerve fiber layer thickness was greater in the AD group on OCT, except within the temporal region, where it was significantly thinner.

Central macular thickness and volume were less in the AD group.5

Anatomic variations can be important. The importance of these findings was underscored by Dr. Girkin. “The size of the nerve has a significant impact in how we diagnose glaucoma,” he said, adding that African-Americans tend to get misdiagnosed with glaucoma possibly because the physician who sees a big cup automatically assumes a diagnosis of glaucoma. “The larger nerve tends to look glaucomatous even if it’s normal,” he said. “But physicians really have to think about the volume and shape of the rim more than the cup. The configuration of rim tissue reflects the amount of neural tissue in the eye, and that is what you’re trying to preserve.”

Another question is why glaucoma is more aggressive in blacks. “We think it’s likely explained by differences in the structure of the optic nerve, specifically the supportive connective tissues that lead to a biomechanical vulnerability to progressive glaucomatous injury,” Dr. Girkin said.
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